FIG. 1 illustrates a block diagram of a conventional light-source driving system 100 for driving a light source 110. The light-source driving system 100 includes a full-bridge rectifier 102, an AC/DC (alternating current to direct current) converter 104, a DC/DC converter 150, a feedback circuit 108, and a controller 112.
The rectifier 102 rectifies an AC voltage VAC (e.g., 220 VAC, 110 VAC, or the like from an electric supply) to provide a rectified power to the AC/DC converter 104. The AC/DC converter 104 converts the rectified power to a DC voltage VDC according a signal 106 from the feedback circuit 108. The signal 106 represents a level of the DC voltage VDC and functions as a feedback to control the AC/DC converter 104 such that the DC voltage VDC is maintained at a constant target voltage level. The DC/DC converter 150 includes an inductor LB, a diode 114, and a switch QSW. When the switch QSW is on, an increasing current can flow through the inductor LB from the AC/DC converter 104 to ground, and the inductor LB stores energy. When the switch QSW is off, the inductor LB releases energy, and the energy released from the inductor LB, together with the power, e.g., the DC voltage VDC, from the AC/DC converter 104, are transformed to an output current IO to flow through the light source 110. The controller 112 can sense the output current IO and generate a control signal 116 to control the switch QSW according to the output current IO, thereby adjusting the output current IO to a target current level.
Due to non-ideality of circuit components in the controller 112 and/or variation in ambient temperature, the output current IO may change even if the output voltage VO of the DC/DC converter 150 remains constant. Thus, the controller 112 controls the DC/DC converter 150 to increase or decrease the output voltage VO, thereby maintaining the output current IO at the target current level. Additionally, light sources having controllable brightness are widely used in many applications. For example, a user may increase or decrease a brightness of the light source 110 by rotating a knob-like switch. The changes in the brightness is caused by increasing or decreasing the output current IO flowing through the light source 110, which can also be caused by increasing or decreasing the output voltage VO. Thus, when the target current level of the output current IO is changed, e.g., by a user, the controller 112 can control the DC/DC converter 150 to increase or decrease the output voltage VO, thereby adjusting the output current IO to the new target current level.
Thus, the light-source driving system 100 includes an AC/DC converter 104 to convert an AC voltage VAC to a constant DC voltage VDC, and includes a DC/DC converter 150 to convert the DC voltage VDC to an adjustable output voltage VO. The DC/DC converter 150 increases or decreases the output voltage VO to adjust the output current IO to a target current level. However, the DC/DC converter 150 consumes power, which increases power consumption and reduces a power efficiency of the light-source driving system 100. The DC/DC converter 150 also occupies a relatively large area of a PCB (printed circuit board), which increases the size of a PCB for the light-source driving system 100. The DC/DC converter 150 also increases the cost of the light-source driving system 100.